In the field of orthopedic joint replacement of the knee, a relatively recent evolution of the art has resulted in prosthetics that require less removal of bone stock as the preparation for the implantation of the prosthetic devices. This trend in technique and prosthetic design is primarily aimed at creating more options for the orthopedic surgeon when challenged with treating a young active individual with joint disease. It has now become well accepted that treating this individual with a total joint arthroplasty (prosthetic) will be an effective way of relieving the symptoms of pain, but this younger patient will likely place demands on his prosthetic joint that will result in the rapid wear, loosening, and need for replacement of the implants.
In the natural knee, loads transmitted to the joint surfaces during normal activities such as running, walking, and jumping create harsh environments for the articular surface tissues. These articular surface tissues consisting of hyaline cartilage play a key role in the load distribution and impact absorption capability of the knee. The wear and degradation of these tissues is typically the endpoint, which creates symptoms of pain and eventually drives an individual to consider joint replacement surgery.
However, many structures and tissues in the joint as well as the musculature of the leg play a complex role in the distribution and management of the loads ultimately seen by the articular surfaces. Arguably one of the most important of these is the meniscus. The meniscus is a kidney-bean shaped structure which attaches to the top articular surface of the tibia with two bony inserts, one anteriorly and one posteriorly. The meniscus is contoured so that it matches the surface of the tibial articular surface on its underside and matches the convex curve of the mating femoral articular surface on its topside. It is comprised of a highly organized system of fibrous bands that are circumferential and give the structure its hoop strength characteristics. In this capacity, the meniscus is understood to significantly increase the contact surface area of the joint so that loads are more evenly distributed over a greater area of both the femoral and tibial hyaline cartilage surfaces. For this reason, there is great advantage to try to preserve this structure.
In prosthetic knee joints, one of the greatest causes of failure is based on wear debris of the tibial component. Not only does this result in the wear of the tibial component but also produces small wear debris particles which stimulate osteolysis, inflammatory changes in neighboring tissues, and eventual loosening of the implant.
Modem prosthetic joint design relies on a careful contour matching of the tibial component, typically composed of ultra-high molecular weight polyethylene (UHMWPE), to the femoral component, typically composed of a Cobalt-Chrome Alloy (CoCr). Mismatch of the surfaces or misalignment of the surfaces during surgical implantation will cause accelerated wear and early failure of the joint. However, as a function of the complex mechanics of the knee, even in the event of perfect surgical matching and alignment of the two components, there are still articulations of the prosthetic joint that create very concentrated local or even point loads between the two components, which result in the creation of a shear particle from the UHMWPE surface.
There would be a great advantage in developing a tibial component prosthetic component that could be implanted without requiring the removal of the meniscus. This component could effectively resurface only a portion of the overall tibial surface, so that the worn exposed portion of the tibial articular cartilage would be replaced, but areas of the tibial surface underlying the meniscus, and the insertion sites of the meniscus would remain intact. This implant would preserve as much normal knee anatomy and load bearing tissue as possible in an attempt to eliminate concentrated loads or point loads between a femoral component (as appears in previous patents) and the described tibial component. A system of instruments useful in locating, positioning and delivering the prosthetic is included.